Low-noise amplifying circuit

ABSTRACT

A low-noise amplifier circuit is specified which has a switchable gain ratio. For this purpose, a parallel circuit comprising a first and a second current path ( 3, 4 ) is provided between a radio-frequency signal input and output ( 1, 2 ), with the first current path ( 3 ) having a transistor which is connected in a common-base circuit for signal amplification, and the second current path ( 4 ) having a transistor which is connected in a common-emitter circuit ( 7 ) for signal amplification, and has input impedance matching ( 25, 27 ). Owing to the good noise characteristics and the good linearity characteristics, the described low-noise amplifier circuit is suitable for use in radio-frequency receivers in which adaptive pre-amplification is required even before a frequency converter, that is to say at the radio-frequency level, because the input signal has a wide dynamic range, such as that in the case of UMTS.

The present invention relates to a low-noise amplifier circuit.

Low-noise amplifiers LNA are normal signal processing blocks forradio-frequency reception.

Mobile radio standards which operate using code-division multiple accessmethods, such as W-CDMA, Wideband Code Division Multiple Access, useradio-frequency signals with a wide dynamic range of, for example, 80dB. In mobile radio receivers such as these, signals with low inputlevels must be amplified using low-noise amplifiers even beforefrequency conversion.

For high reception levels, excessive gain would lead to overdriving ofthe subsequent stages in the signal processing chain of the receiver andto contravention of the linearity requirement in the respective mobileradio standard. It is therefore desirable to design low-noise amplifierswhich have a variable gain factor.

Further requirements which are applicable to an amplifier that issuitable for the methods which have been mentioned are a high adjustablegain in order to obtain good noise characteristics overall, as well ascompliance with the strict linearity requirements, which are notconsistent with high gain. Furthermore, good input-side matching must beprovided in terms of both the power and the noise in order to achievegood matching to the output impedance of an upstream filter when thelevels to be expected are low. If a mixer that is connected downstreamfrom the LNA is not in the form of a mixer which suppresses mirrorfrequencies, then the LNA must have good backward isolation, in order tosuppress leakage frequencies from the local oscillator signal which canbe supplied to the mixer. Since mobile radio receivers in the describedcategory are normally used not only in fixed stations, but also inmobile stations, attention should also be paid to low power consumption.

In the case of UMTS (Universal Mobile Telecommunications Standard)appliances, in which the noise factor over the entire receiver chainmust not exceed 8 dB, from which the expected insertion loss of a filterof 3 dB and the downstream stages, likewise of 3 dB, must be subtracted,this means that the desired low-noise amplifier should guarantee a noisefactor of <2 dB with a high gain of >15 dB.

Section B.) of the document J. R. Long “A Low-Voltage 5.1–5.8-GHzImage-Reject Downconverter RF IC”, Journal of Solid-State Circuits, Vol.35, No. 9, September 2000, pp. 1320–1328 describes how good noisematching and power matching can be achieved at the same time for anamplifier in which a transistor is operated in a common-emitter circuit.Even if the amplifier is operated with inductive degeneration, that isto say the emitter inductance with respect to the reference-groundpotential, with a collector load in the form of a resonant circuit aswell as a cascode circuit, a particularly high current density isrequired in the input transistor in order to achieve adequate linearity.In UMTS receivers, LNAs are operated with low gain for 80% of the time,so that the problem of power wastage is further exacerbated.

In addition, a circuit implementation which is feasible in this case inorder to achieve a switchable gain by derivation of a part of the outputcurrent with respect to the supply voltage leads only to aninsignificant improvement in the linearity while, in contrast, the noisein the amplifier increases severely.

The described problems could be overcome by using a circuit with atransistor which is connected in a common-emitter circuit when thedesired LNA gain is high, and by using a low LNA gain, with a separateamplifier stage with a transistor which is connected in a common-basecircuit.

Interconnection such as this results, however, in the problem that thetwo amplifiers in the common-base and common-emitter circuits have adifferent input impedance, which prevents noise and power matching bymeans of a common matching network.

The object of the present invention is to specify a low-noise amplifierwhose capability to adjust the gain means that it is suitable forcode-division multiple access methods and which offers good noise andpower matching capability.

According to the invention, the object is achieved by a low-noiseamplifier circuit, having

-   -   a signal input for supplying a radio-frequency signal,    -   a signal output for producing an amplified signal which is        derived from the radio-frequency signal,    -   a first current path, which couples the signal input to the        signal output and has a transistor connected in a common-base        circuit,    -   a second current path, which couples the signal input to the        signal output and has a transistor connected in a common-emitter        circuit, with the transistor which is connected in a        common-emitter circuit being followed by a further transistor        connected in a common-base circuit in order to form a cascode        circuit, and with the transistor which is connected in a        common-emitter circuit having a feedback path between its        collector and base connections, as well as a resistor which        couples the collector connection of the transistor which is        connected in a common-emitter circuit to the emitter connection        of the further transistor which is connected in a common-base        circuit, and    -   a switching device for activation of the first or second signal        path as a function of the desired gain.

The described LNA (Low Noise Amplifier) structure offers the capabilityto switch between two fixed gain ratios and is thus in principlesuitable for amplification of W-CDMA signals with a wide dynamic range.The LNA can therefore be used in receivers for the UMTS mobile radiostandard.

The signal path with the transistor which is connected in acommon-emitter circuit operates with high gain and with very goodefficiency, and thus complies with the linearity, gain and noiserequirements that are applicable in this amplification range. Thecurrent path with the transistor which is connected in a common-basecircuit is provided for low gain levels, allowing high linearity to beachieved with low gain and with a low power consumption. The inputimpedance is in this case virtually real and corresponds to thereciprocal of the gradient and, furthermore, the backward isolation isvery good. The effects of the relatively poor noise characteristics ofthe common-base circuit are minor since the common-base circuit is usedfor low gain so that high input levels are therefore in any case presentat the signal input. The signal-to-noise ratio, SNR, is thereforereduced only slightly.

The feedback between the collector and base connection of the transistorwhich is connected in a common-emitter circuit in the second currentpath results in transconductance, that is to say compensation for theinput impedance of the emitter circuit, which is otherwise highlycapacitive. The series resistance which is also provided for couplingthe cascode transistor to the transistor which is connected in acommon-emitter circuit also provides voltage amplification, whichfurther improves the linearity, noise and efficiency of the circuit.

Since the common-emitter circuit has an inverting behavior, the feedbackbetween the collector and the base of the transistor may be in the formof capacitive feedback, which acts like an inductance and thuscompensates for the actually capacitive input impedance to form avirtually real input impedance.

In consequence, both the first and the second current path with thetransistor which is connected in a common-base circuit and thetransistor which is connected in a common-emitter circuit each have avirtually real input impedance, which provides simple noise and powermatching to an upstream stage.

Since this therefore avoids the use of an additional on-chip inductance,which would consume a relatively large amount of chip area, this on theone hand makes it possible to provide a circuit with a small chip areawhile, on the other hand, this avoids interference coupling resultingfrom the large chip area of an additional inductance.

In one preferred embodiment of the present invention, the feedback pathof the transistor which is connected in a common-emitter circuit has aresistor and capacitance connected in series.

In principle, any desired combination of resistances, capacitances andinductances may be used in the capacitive feedback path, but a seriescircuit comprising resistance and capacitance leads to particularly goodresults with regard to noise characteristics and gain.

In a further preferred embodiment of the present invention, the firstand the second current path, as well as the signal input and the signaloutput of the LNA, are based on symmetrical circuitry.

The symmetrical circuitry for carrying differential signals offers ahigh degree of insensitivity to interference in the amplifierarrangement and, furthermore, allows simple connection to a downstreamfrequency mixer without any additional circuitry measures. Furthermore,symmetrical circuitry means that there are no sudden phase changes whenswitching the gain, as would otherwise occur as a result of theinverting behavior of the common-emitter circuit and the non-invertingbehavior of the common-base circuit. The transistors which are describedin the first and second current paths each have to be provided in aduplicated form, corresponding to the symmetrical circuitry. The twotransistors which are connected in a common-emitter circuit in thesecond current path in this case form a differential amplifier, with theemitter connections being coupled.

In a further preferred embodiment of the present invention, the firstcurrent path has a cascode stage which is connected downstream from thetransistor which is connected in a common-base circuit. This makes itpossible to increase the backward isolation.

In a further preferred embodiment of the present invention, theswitching device has a first switch, which is connected to a controlinput of the transistor which is connected in a common-base circuit, forconnection/disconnection of a first bias voltage, and has a secondswitch, which is connected to a control input of the further transistorwhich is connected in a common-base circuit in the second current path,for connection/disconnection of a second bias voltage.

In addition to the described switching between high and low gain in theLNA, the current sources which supply the current paths can preferablybe connected and disconnected. In addition to better noisecharacteristics, this also makes it possible to reduce the powerrequirement.

In a further preferred embodiment of the present invention, a resonantcircuit is provided which couples the two current paths on the signaloutput side to a supply potential connection and may be designed to havea narrow bandwidth. The tunable coupling via a system which canoscillate, that is to say a tank, leads on the one hand to the avoidanceof a DC voltage drop from the supply voltage and thus to betterutilization of the voltage in the amplifiers while, on the other hand,the resonant circuit allows matching to normally capacitive loadswithout any additional complexity. The narrowband resonant circuit maybe formed, for example, with a coil with a center tap for connection tothe supply voltage and capacitances. Furthermore, the narrowbandresonant circuit results in a slight improvement in selectivity in theamplifier.

In a further preferred embodiment of the present invention, thetransistor which is connected in a common-emitter circuit in the secondcurrent path is provided with an inductance which couples the emitterconnection of the transistor to a reference-ground potential connection.

This so-called inductive degeneration leads to better linearitycharacteristics and to a better matching capability for the inputimpedance with regard to power and noise.

In a further preferred embodiment of the present invention, a seriescapacitance is provided for coupling the signal input and the firstcurrent path. Instead of the series capacitance, other components with ahigh-pass filter characteristic may also be provided.

In a further preferred embodiment of the present invention, a firstcurrent source is provided for supplying the first current path and iscoupled, such that it can be connected/disconnected, to the transistorwhich is connected in a common-base circuit.

In a further preferred embodiment of the present invention, a secondcurrent source is provided for supplying the second current path and iscoupled, such that it can be connected/disconnected and via a currentmirror transistor, to the second current path.

The current mirror transistor may in this case preferably be connectedvia resistors to the base connections of the transistors which areconnected in a common-emitter circuit.

Further details and embodiments of the invention are the subject matterof the dependent claims.

The invention will be explained in more detail in the following textusing an exemplary embodiment and with reference to the drawings, inwhich:

FIG. 1 shows a first exemplary embodiment of a low-noise amplifieraccording to the invention, designed using symmetrical circuitry, and

FIG. 2 shows an S-parameter diagram in order to illustrate the inputimpedance matching according to the invention.

FIG. 1 shows a low-noise amplifier designed using analog, bipolarcircuitry and with signals being carried symmetrically. The low-noiseamplifier LNA has a symmetrical signal input 1 and a symmetrical signaloutput 2, at which an amplified signal can be tapped off.

Two parallel current paths 3, 4 are provided between the signal input 1and the signal output 2, with a switching capability being providedbetween the current paths 3, 4 in order to achieve a switchable-gainLNA.

The first current path 3 has two transistors 5, 6 which are connected ina common-base circuit, while the second current path 4 has two bipolartransistors 7, 8, which are operated in a common-emitter circuit andform a differential amplifier stage. The second current path 4 isactivated in order to achieve high gain, while low gain levels, that isto say when high signal levels are present at the input 1, are producedby the first current path 3.

In detail, the first current path 3 has the two bipolar transistors 5,6, whose base connections are connected to one another and can beconnected via a switch 9, such that they can be connected to a fixedbias voltage source. The connection for supplying the bias voltage isannotated 10. The emitter connections of the transistors 5, 6 arecoupled via a respective series circuit comprising a capacitance 11, 12and a resistor 13, 14 to the symmetrical signal input 1, with thecapacitances 11, 12 being connected upstream of the resistors 13, 14 inthe signal transmission direction. A respective current source 15, 16,which is connected to a reference-ground potential, is also in each caseconnected via a respective further switch 17, 18 to the emitterconnections of the transistors 5, 6. The collector connections of thetransistors 5, 6 are coupled to the signal output 2, with the signallines for carrying the differential signal in the second current pathbeing routed such that they are crossed over in order to prevent asudden phase change when switching between the first and the secondcurrent path. A cascode stage 19 is optionally provided between thecollector connections of the transistors 5, 6 and the signal output 2.The switches 9, 17, 18 must be closed in order to activate the firstcurrent path, in order on the one hand to supply a constant bias voltageto the base connection of the transistors 5, 6 and, on the other hand,supply the current, which must be provided for the current gain, on theemitter side. When the first current path 3 is not activated, that is tosay when the higher gain is selected, the switches 9, 17, 18 are opened.In the last-mentioned state, the output side of the switch 9 isconnected to a reference-ground potential connection.

The second current path 4 has a differential amplifier with a cascodecircuit and includes the two transistors 7, 8, which are operated in acommon-emitter circuit, as well as two further transistors 20, 21, whichare operated in a common-base circuit. In order to form the differentialamplifier circuit with a cascode stage, the emitter connections of thetransistors 20, 21 are coupled to a respective collector connection ofthe transistors 7, 8. The base connections of the transistors 7, 8,which are operated in a common-emitter circuit, are connected to thesymmetrical signal input 1.

The emitter connections of the transistors 7, 8 are connected to areference-ground potential connection 24 via a respective inductance 22,23. Feedback impedances 25, 26 are provided for matching the capacitiveinput impedance of the second current path 4 with the differentialamplifier which is connected in a common-emitter circuit, andrespectively connect the base connection and collector connection of thetransistors 7, 8 to one another. In order to provide additional voltagegain, respective resistors 27, 28 are provided between the emitterconnections of the transistors 20, 21 and the collector connections ofthe transistors 7, 8.

The base connections of the cascode transistors 20, 21 are connected toone another, and are connected via a switch 29 to a connection 30 forsupplying a second bias voltage. The switch 29 is closed in order toactivate the higher gain, while the switch 29 is opened duringactivation of the first current path 3, thus connecting the input 30 tothe reference-ground potential connection 24. The impedances 25, 26 inthe present embodiment are in the form of a series circuit comprising acapacitance 41 with a resistor 40.

In order to supply current to the differential amplifier 7, 8 in thesecond current path 4, the transistors 7, 8 each form a current mirrorwith a current mirror transistor 31. The current mirror transistor 31 isconnected to the reference-ground potential connection 24 via itsemitter connection. A current source 33, which is connected to a supplypotential connection 34, is connected via a switch 32 to the collectorconnection of the current mirror transistor 31. The base connection ofthe current mirror transistor 31 is connected via a resistor 35 to itscollector connection. This collector connection is connected via arespective resistor 36, 37 to a base connection of the respectivetransistors 7, 8.

The first and second current paths 3, 4 are also connected on the signaloutput side, that is to say at that symmetrical circuit node whichconnects the two current paths to one another and to the signal output2, to the supply potential connection 34, via a narrowband resonantcircuit 36.

The resistors 27, 28 allow sufficiently high voltage gain in order toachieve input impedance matching by means of the impedances 25, 26 viavoltage/current feedback, that is to say shunt—shunt feedback, such thatthe normally capacitive input impedance of the second current path 4becomes virtually purely real, and thus corresponds to the inputimpedance of the transistors which are connected in a common-basecircuit in the first current path. This allows the circuit asillustrated in FIG. 1 to be matched in a simple manner in terms of powerand noise to an upstream stage, for example a filter. On the outputside, a downward mixer in a mobile radio receiver may be connected, forexample, to the circuit shown in FIG. 1.

The switching between high and low gain allows narrowbandradio-frequency signals, which are coded using a code-division multipleaccess method and have a wide dynamic range, to be pre-amplified beforedownward mixing, with little noise, a high gain, with a low signallevel, and with good linearity characteristics overall. Furthermore, thedescribed LNA has good backward isolation and is thus suitable forsuppression of local oscillator leakage frequencies even in homodynereceivers which do not have mirror-frequency suppressing filters. Thesymmetrical design of the described LNA offers a high degree ofinsensitivity to interference. Overall, the circuit as shown in FIG. 1can be operated with a low power requirement. This allows it to be usedin mobile appliances, for example mobile stations, which operate inaccordance with the UMTS Standard. The use of the invertingcharacteristics means that there is no need for any inductances to formthe impedances 25, 26 in the second current path 4 in the feedback pathfor input impedance matching, so that the circuit can be formed with asmall area and with low sensitivity to interference.

The circuit shown in FIG. 1 with a first and a second current path canbe matched by means of just one matching network to an upstream stage,for example a filter.

FIG. 2 uses a diagram based on an S parameter representation to show howthe circuit illustrated in FIG. 1 operates. The point that is providedwith the reference symbol 37 denotes the input impedance of thecommon-base circuit in the first current path 3. The reference symbol 38denotes the input impedance of the differential amplifier 7, 8 withoutany feedback with the elements 25 to 28, while the reference symbol 39denotes the matched input impedance, as illustrated in FIG. 1. As can beseen, the input impedances 37, 39 are sufficiently close to one anotherthat the two amplifier stages can now be matched using the same matchingnetwork. FIG. 2 also shows that matching to a previous stage would bepossible even without any additional elements. However, an externalnetwork simplifies the matching between the noise behavior, thelinearity and the gain.

LIST OF REFERENCE SYMBOLS

-   1 Signal input-   2 Signal output-   3 First current path-   4 Second current path-   5 Transistor-   6 Transistor-   7 Transistor-   8 Transistor-   9 Switch-   10 Supply voltage supply connection-   11 Capacitor-   12 Capacitor-   13 Resistor-   14 Resistor-   15 Current source-   16 Current source-   17 Switch-   18 Switch-   19 Cascode stage-   20 Transistor-   21 Transistor-   22 Inductance-   23 Inductance-   24 Reference-ground potential connection-   25 Impedance-   26 Impedance-   27 Resistor-   28 Resistor-   29 Switch-   30 Supply voltage supply connection-   31 Current mirror transistor-   32 Switch-   33 Current source-   34 Supply potential connection-   35 Resistor-   36 Resonant circuit-   37 Input impedance-   38 Input impedance-   39 Input impedance-   40 Resistor-   41 Capacitor

1. A low-noise amplifier circuit, comprising: a signal input forreceiving a radio-frequency signal; a signal output for providing anamplified signal which is derived from the radio-frequency signal; afirst current path coupled between the signal input and the signaloutput, said first current path including a first transistor connectedin a common-base circuit; a second current path coupled between thesignal input and the signal output, said second current path including asecond transistor connected in a common-emitter circuit, the secondtransistor having a collector and a base, and including a feedback pathcoupled between the collector and the base of the second transistor; athird transistor connected in a common-base circuit; a resistor coupledbetween the collector of the second transistor and an emitter of thethird transistor; and a switching device comprises a plurality ofswitches for selectively activating one of the first and second currentpaths as a function of a desired gain.
 2. The amplifier circuit of claim1, wherein the feedback path includes a resistor and a capacitanceconnected in series.
 3. The amplifier circuit of claim 2, including aresonant circuit coupled to the signal output and the first and secondcurrent paths, the resonant circuit adapted for connection to a supplypotential.
 4. The amplifier circuit of claim 3, including an inductancefor coupling the emitter of the second transistor to a referencepotential.
 5. The amplifier circuit of claim 2, including an inductancefor coupling the emitter of the second transistor to a referencepotential.
 6. The amplifier circuit of claim 1, wherein the signalinput, the signal output and the first and second current paths are forcarrying differential signals.
 7. The amplifier circuit of claim 6,including a resonant circuit coupled to the signal output and the firstand second current paths, the resonant circuit adapted for connection toa supply potential.
 8. The amplifier circuit of claim 7, including aninductance for coupling the emitter of the second transistor to areference potential.
 9. The amplifier circuit of claim 6, including aninductance for coupling the emitter of the second transistor to areference potential.
 10. The amplifier circuit of claim 1, wherein thefirst current path includes a cascode stage connected between the firsttransistor and the signal output.
 11. The amplifier circuit of claim 10,including a resonant circuit coupled to the signal output and the firstand second current paths, the resonant circuit adapted for connection toa supply potential.
 12. The amplifier circuit of claim 11, including aninductance for coupling the emitter of the second transistor to areference potential.
 13. The amplifier circuit of claim 10, including aninductance for coupling the emitter of the second transistor to areference potential.
 14. The amplifier circuit of claim 10, wherein theswitching device includes a first switch connected to the firsttransistor for selectively connecting a first bias voltage to the firsttransistor, and a second switch connected to the third transistor forselectively connecting a second bias voltage to the third transistor.15. The amplifier circuit of claim 14, including a resonant circuitcoupled to the signal output and the first and second current paths, theresonant circuit adapted for connection to a supply potential.
 16. Theamplifier circuit of claim 15, including an inductance for coupling theemitter of the second transistor to a reference potential.
 17. Theamplifier circuit of claim 14, including an inductance for coupling theemitter of the second transistor to a reference potential.
 18. Theamplifier circuit of claim 1, including a resonant circuit coupled tothe signal output and the first and second current paths, the resonantcircuit adapted for connection to a supply potential.
 19. The amplifiercircuit of claim 18, including an inductance for coupling the emitter ofthe second transistor to a reference potential.
 20. The amplifiercircuit of claim 1, including an inductance for coupling the emitter ofthe second transistor to a reference potential.
 21. The amplifiercircuit of claim 1, including a capacitance coupled in series betweenthe signal input and the first current path.
 22. The amplifier circuitof claim 1, including a first current source coupled to the firstcurrent path for feeding the first current path, the first currentsource selectively connectable to the first transistor.
 23. Theamplifier circuit of claim 22, including a second current source forfeeding the second current path, and a current mirror transistor forcoupling the second current source to the second current path, thesecond current source selectively connectable to the second transistorvia the current mirror transistor.
 24. The amplifier circuit of claim 1,including a second current source for feeding the second current path,and a current mirror transistor four coupling the second current sourceto the second current path, the second current source selectivelyconnectable to the second transistor via the current mirror transistor.25. A low-noise amplifier circuit, comprising: a signal input forreceiving a radio-frequency signal; a signal output for providing anamplified signal which is derived from the radio-frequency signal; afirst current path coupled between the signal input and the signaloutput, said first current path including a first transistor connectedin a common-base circuit; a second current path coupled between thesignal input and the signal output, said second current path including asecond transistor connected in a common-emitter circuit, the secondtransistor having a collector and a base, and including a feedback pathcoupled between the collector and the base of the second transistor; athird transistor connected in a common-base circuit; a resistor coupledbetween the collector of the second transistor and an emitter of thethird transistor; and a switching circuit coupled to the firsttransistor, and configured to selectively connect a first bias voltageto the first transistor, and coupled to the third transistor andconfigured to selectively connect a second bias voltage to the thirdtransistor, in accordance with a desired gain.